Correlation is not causation. If anything, it's the other way around: being fat when you'd rather not be makes you more likely to choose a diet drink over a sugary one. Some legit problems might be found in artificial sweeteners like maybe gut microbiome disruption and stuff, but if I want something sweet I'd take even aspartame (which I avoid) over real sugar because fake sweeteners might turn out to be a problem but I know for sure sugar is for me. I can down an entire liter of artificially sweetened sparkling water drink with no detectable reaction, but when I ate even one tiny clementine orange or slice of watermelon, my body immediately went, "That's real sugar! Where's the rest of the oranges/melon?!!!" And had to use willpower to make myself not take another piece instead of diving face-first into the fruit bowl. I was a bit surprised at how quickly/easily my body could detect the difference even though both taste sweet. Artificial sweeteners also helped me delay meals during intermittent fasting better than plain water so I lost more weight. Not suggesting they're healthy by any means, but since I can't safely have caloric sweets, even natural stuff like fruits/honey, I'd rather use fake stuff than never have something sweet again unless safety research on various products suggests otherwise.